1. Field of the Invention
The present invention relates to a solvent composition useful for liquid delivery chemical vapor deposition of metal organic precursors including metal (beta-diketonato) precursors.
2. Description of the Related Art
In the liquid delivery method of carrying out chemical vapor deposition (CVD) processes, a solid precursor is dissolved in an appropriate solvent medium or a liquid-phase precursor is vaporized and the resulting precursor vapor, typically mixed with a carrier gas (such as argon, helium or nitrogen) is transported to the chemical vapor deposition reactor. In the reactor, the precursor vapor stream is contacted with a heated substrate to effect decomposition and deposition of a desired component or components from the solution and/or vapor phase on the substrate surface.
In such liquid delivery CVD process, a wide variety of solvents have been employed for dissolution or suspension of precursor species, with the liquid solution or suspension being vaporized by various techniques, including flash vaporization on a heated element onto which the liquid containing the precursor is discharged, to volatilize the solvent and precursor species.
In many instances, where a variety of precursors are employed to form a multi-component deposited film in the CVD process, it is desirable to utilize a single solvent medium for the respective precursor species, for ease of operation and simplicity of the process system, thereby avoiding any deleterious solvent-solvent interactions which may occur if different solvent media are utilized for different precursor species. Further, it is desirable that solvent compositions when used for multiple species not interact with the precursor or metal-containing molecules to form unstable chemical solutions, since such instability renders the overall composition unsuitable for liquid delivery.
In a specific field in which the present invention has applicability, ferroelectric ceramic materials based on bismuth oxide are promising materials for use in non-volatile memories. Promising candidates derive from the group of Aurivillius phase compounds having the general formula:
(Bi.sub.2 O.sub.2).sup.2+ (A.sub.m-1 B.sub.m O.sub.3m+1).sup.2-, wherein A=Bi.sup.3+, L.sup.3+, L.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Pb.sup.2+, Na.sup.+, B=Fe.sup.3+, Al.sup.3+, Sc.sup.3+, Y.sup.3+, L.sup.4+, Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, W.sup.6+, Mo.sup.6+, with L=metal from the lanthanide series, such as Ce.sup.4+, La.sup.3+, Pr.sup.3+, Ho.sup.3+, Eu.sup.2+, Yb.sup.2+, etc. and m=1,2,3,4,5.
Among materials of the foregoing type, SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT) and Bi.sub.4 Ti.sub.3 O.sub.12 find widespread interest for integration in ferroelectric random access memories (FeRAMs) and in smart cards.
In chemical vapor deposition processes for SBT, the use of precursors such as Sr(thd).sub.2 (tetraglyme), Ta(OiPr).sub.4 (thd) and triphenyl bismuth, dissolved in a solvent mixture such as tetrahydrofuran: isopropanol: tetraglyme in a volumetric ratio of 8:2:1 produced the result that Bi.sub.2 O.sub.3 deposition was difficult to control. Efforts to resolve such difficulties included replacement of the triphenyl bismuth precursor with Bi(thd).sub.3 with the latter precursor showing a reliable and reproducible Bi.sub.2 O.sub.3 deposition rate. Unfortunately, however, in the vaporizer the Bi(thd).sub.3 precursor caused the formation of black bismuth-rich residues, indicating premature decomposition was taking place during vaporization and transport. Such premature decomposition allowed only ten operational runs to be conducted with the Bi(thd).sub.3 precursor until the vaporizer required maintenance to remove unwanted deposits.
Chemical considerations associated with the foregoing adverse decomposition indicated the solvent system was one source of the problem. It appeared that the isopropanol was reacting with the Bi(thd).sub.3 precursor in the elevated temperature conditions of the vaporizer (190.degree. C.) and the precursor was resultingly reduced to Bi metal producing the black residue. Concurrently, it is expected that the IPA is oxidized during this decomposition (redox) reaction.
Accordingly, an improved solvent system is desired for such deposition process for the formation of SBT films. Such a solvent system faces a number of problems. The solubility of the precursors in the solvent medium must be sufficiently high to provide adequate precursor delivery rates in the vaporizer. Moreover, there should not be a precipitation of material over a period of time due to any slight oversaturation incurred during preparation of the source reagent compositions, or caused by a ligand exchange among the different precursor species.
It therefore is the object of the present invention to provide a novel solvent composition having broad utility for CVD precursors, such as those comprising metal organic compositions with .beta.-diketonate ligands.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.